1. Field of the Invention
The present invention relates to air conditioning units mounted on the roof tops of buildings. More specifically, the field of the invention is that of base assemblies for roof top air conditioners.
2. Prior Art.
Many buildings, particularly small and medium sized commercial buildings, have flat roofs on which air conditioning units are located. These buildings include curb structures which both support the large and heavy machinery of the air conditioning unit, and connect the air conditioning unit with air passages and electrical lines of the building. Rooftop air conditioners generally include an air conditioning unit disposed within a cabinet or housing, with the cabinet or housing supported on the curb of the flat roof deck. Base assemblies may provide support for the air conditioning unit, and are generally rectangular for supporting rectangular base pans disposed within the base assembly and on the roof curb. Many base assemblies are disposed around the perimeter of the roof curb which supports both the base pans and the base assembly, although some base assemblies support the base pans above the roof curb.
Base assemblies which are peripherally disposed around the curb do not need to support the weight of the air conditioning unit because it is mainly supported by the roof curb. For such peripherally disposed air conditioners, structural stress is applied to the base assembly primarily during storage, shipment, and installation. During storage and shipment of the air conditioner, the load on the base assembly is the entire weight of the air conditioning unit, a much greater load than the load on the base assembly after the unit is installed. In regards to installation, the peripherally disposed base assembly still must provide a structure which facilitates the attachment of the rooftop air conditioner unit to the delivery vehicle, which is typically a crane or helicopter. Therefore, the structural strength of such a base assembly is most important during the storage, shipment, and installation of peripherally disposed rooftop air conditioners.
The cost of delivering the air conditioner is another factor in the total cost of a rooftop unit, and the delivery or shipping expense increases with the shipping weight of the unit. Rooftop air conditioners are generally transported with additional materials that protect the unit during transportation and facilitate installation. These additional protective materials add to the shipping weight of the unit, and require additional time and effort to remove during installation. Therefore, one desirable feature of a rooftop air conditioner is a relatively low shipping weight because the delivery cost of the unit increases in proportion to its shipping weight.
Prior art base assemblies are generally formed of four side rails joined together at their corners by welding or similar process to form a seam on the vertical web between the adjoining side rails. This vertical seam is not as structurally strong as the vertical web and thus may deform after prolonged stress. Further, to form such a corner by welding or the like, a relatively heavy gauge metal must be used. However, the manufacturing cost includes the cost of the welding process and the required additional materials, and further contributes to the weight of the air conditioner unit and therefore increases the delivery costs.
Because of the rectangular shape, prior art base assemblies are formed from at least two metal portions having different lengths. This requires at least two different lengths of the material which forms the side portions. Further, the dimensions of the base assembly cannot be changed except by cutting off part of the length of two opposite side portions. Having unequal lengths of side portions tends to increase the amount o scrap metal formed during manufacture, and thus the cost of materials is increased. Also, the additional step of adjusting the length of the side portions by physically alteration further increases manufacturing costs.
Another potential problem with prior art structures involves the manufacturing tolerances of the base pans. Typically, two or three base pans are located end to end and define the dimensions of the base assembly. Base pans are generally manufactured within preset tolerances, for example wherein the length and width of the base pan is within .+-.1 mm of the predetermined dimensions. In rooftop air conditioners, multiple base pans are combined together to form a larger rectangular outline, typically having three base pans. This results in the variation in length of the base assembly, for example, to be multiplied by three to .+-.3 mm wherein the variation of the width is within .+-.1 mm. In attaching the base pans to the base rails to form the base assembly, tolerances may be accommodated if they are within accepted ranges. However, if the cumulative variation is too large, the overall strength of the base assembly may be adversely effected.
In the aforementioned example, the variation of .+-.1 mm is acceptable, but the variation of .+-.3 mm is not acceptable. This is due to the ability of a preformed component, the base pan, to be able to accommodate the 1 mm difference and interfit with the other component, the base assembly, without losing significant structural strength. On the other hand, the same component may not be able to accommodate a 3 mm difference without adversely effecting its structural strength. The steel sheets which comprise the base side portion and base pan components may be bent and reformed, but each deformation of the sheet metal weakens its structural integrity. Thus by bending the sheet metal slightly, the structure is slightly weakened, and bending the sheet metal significantly may cause significant and unacceptable weakening of the sheet metal.
To avoid the problems inherent with too large of a variation, two approaches may be taken. For one, the base side portions may be formed to accommodate the largest variation and then further modified during manufacturing if the actual size needed is smaller. However, a problem with this approach of accommodation is that it involves additional manufacturing steps of measuring the actual length needed and cutting the base side portions to the length needed, both of which add to the manufacturing cost. Alternatively, the base pans may be manufactured more precisely to decrease the variation of the base pans. A problem with this approach is that manufacturing the base pans more precisely increases the expense. Thus, either approach increases the cost of manufacturing the air conditioning unit.
Prior art rooftop air conditioner units typically are moved from their place of manufacture in a shipping package which helps support the unit during transportation and installation. However, once the air conditioner is installed, the shipping package is no longer needed and often must be removed from the rooftop. Although the shipping package is necessary for lifting, maneuvering, and installing the air conditioner, removing the air conditioner from the shipping package an removing the shipping package from the rooftop adds to the cost of delivering the air conditioner unit.
What is needed is a base assembly which has structural strength to support the air conditioner during storage, shipment, and installation.
Also needed is a base assembly which minimizes its weight.
Another needs exists for a base assembly having stronger corners.
A further need is a base assembly having base rails formed of sheet metal pieces having uniform length.
A still further need is a base assembly which is easily adaptable to manufacturing variations of the base pans.